LTE Advanced is being considered for deployment in the unlicensed spectrum in the 5 GHz band. To do so, licensed spectrum (e.g., in the range 400 MHz to 3.8 GHz) and spectrum in unlicensed spectrum bands would be aggregated together in the same way that the licensed spectrum bands are treated in standard LTE Advanced carrier aggregation (CA). Regulatory authorities around the world have defined or are defining a wide variety of requirements so that various systems can inter-operate or coexist with incumbent users in the unlicensed bands, notably including WiFi deployments.
For LTE Advanced (LTE Release 10-12) carrier aggregation (CA) or Dual Connectivity, an eNB (E-UTRAN Node B or “evolved Node-B”) may be used. An eNB is connected to a mobile phone network that communicates with mobile handsets (i.e., “user equipment” or UEs) and that configures a secondary serving cell (Scell) to the UE to provide additional frequency resources (i.e., a secondary carrier or a secondary component carrier (CC)) for communication in addition to the primary serving cell (Pcell). The Scell is typically activated for a UE at the onset of a data burst transmission for the UE and deactivated after the transmission is complete. Activation is done by using an activation command MAC layer control element (MAC CE). Deactivation is done either upon expiry of a deactivation timer or by using an explicit deactivation command MAC CE. Additionally, a UE operating in an Advanced LTE (LTE Release 12) network can expect at least discovery signals from the eNB on the Scell carrier while the carrier is deactivated. After receiving an activation command for the Scell, the UE expects cell specific reference signal (CRS) transmissions from the eNB on the Scell carrier beginning during the subframe in which the activation command is received until the subframe in which the Scell is deactivated.
For further Advanced LTE (LTE Release 13) Scell, operation on unlicensed carriers continues to be studied. The initial focus of the studies is Scell operation via a CA mechanism. However, it has been perceived that some of the procedures identified for CA can also be reused for dual connectivity (i.e., when the Scell and Pcell belong to different cell groups).
Due to regulatory requirements, and due to the need to for LTE Advanced to co-exist with other wireless systems (e.g., Wi-Fi), LTE devices (i.e., UEs and eNBs) need to take several issues into account while operating on an unlicensed carrier frequency (i.e., using License Assisted Access LTE, or “LAA LTE”).
First, before transmitting on an unlicensed carrier, the LTE devices (e.g., eNB) typically have to check whether the carrier (i.e., the carrier frequency) is busy using some form of ‘listen before talk’ (LBT) mechanism, then an LTE device can begin transmissions only if the carrier is free. LBT typically includes measuring the energy on the carrier (sometimes referred to as sensing) for a short duration (e.g., 9 us or 20 us) and determining whether the measured energy is less than a threshold (e.g., −82 dBm or −62 dBm). If the energy is less than the threshold, the carrier is determined to be free. Some examples of LBT include the CCA-ED (clear channel assessment-energy detect) and CCA-CS (clear channel assessment-carrier sense) mechanisms defined in IEEE 802.11 specifications, and CCA mechanisms specified in ETSI EN 301 893 specification.
Second, transmissions on the carrier typically also have to follow discontinuous transmission requirements (DCT requirements), i.e., the LTE device can continuously transmit for only Xms (e.g., X=4 ms as per regulations for some regions, and up to 13 ms per regulations for other regions), after which it has to cease transmission for some duration (sometimes referred to as the idle period), then perform LBT, and reinitiate transmission only if the LBT procedure is successful (i.e., the LBT procedure indicates that the carrier is not busy). The device may perform LBT towards the end of the idle period.
As a result, mechanisms are needed to enable more efficient wireless network operation in both the licensed and unlicensed carrier spectrums.